License in the Mac installer

[Manuel M T Chakravarty]

Thorkil Naur:
> On Tuesday 12 February 2008 06:56, Manuel M T Chakravarty wrote:
>> Mac installer packages usually present a license to the user to  
>> accept
>> during the installation process.  Consequently, I added what I think
>> is a correct licensing document to the tree at
>>
>>   http://darcs.haskell.org/ghc/distrib/MacOS/installer-docs/
>> license.html
>
> I have reviewed this briefly. It is not the GPL.

It is three licences.  The BSD3-style GHC license, the LGPL, and the  
GPL.

> Elsewhere
> (http://www.haskell.org/pipermail/glasgow-haskell-users/2008-February/014298.html 
> )
> you have written:
>
>> The GHC binary in the package links statically against GNU  
>> readline ...
>
> The GNU readline package is under the GPL. I dont understand how you  
> can link
> statically against the GNU readline library and not put the combined  
> package
> under the GPL.

First of all, whether we link statically or dynamically against  
readline makes no difference whatsoever.  (It's only the LGPL that has  
a special provision for dynamic linking.)

The way I view this is that licences form a partial order, which we  
may call "restrictiveness".  If license A (eg, GPL) is more  
restrictive than license B (eg, BSD3), then a combined product must  
satisfy the conditions of the more restrictive licence.  Of course,  
there are license which are incomparable under this PO and these  
license are incompatible; ie, you can not combine code under these two  
licenses.  (Here is the FSFs definition of what it means for two  
licences to be compatible: <http://www.fsf.org/licensing/licenses/gpl-faq.html#WhatIsCompatible 
 >.)

The important point here is that by offering code under a particular  
licence you implicitly also offer it under all strictly more  
restrictive licenses; i.e., if I give you code under BSD3, you can re- 
distribute it under the GPL.  (If you don't modify it, there is little  
point, because people can get that same code from me under BSD3, but  
legally you can just change the license and not the code.)  This  
ability to morph BSD3 into GPL is what justifies the license  
information in the installer package.  By offering GHC under BSD3, we  
do allow people to distribute it under the GPL and hence fullfil our  
obligations under the GPL (which we incurred by linking - in whatever  
way - against readline).  Does that clarify the point?

Manuel